Instructional Algorithms Enhance Student Understanding of Plc Ladder Logic Programming
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AC 2010-1983: INSTRUCTIONAL ALGORITHMS ENHANCE STUDENT UNDERSTANDING OF PLC LADDER LOGIC PROGRAMMING James Rehg, (Retired) James A. Rehg is a freelance writer and co-owner of Text On Demand, a company specializing in information presentation and documentation. He retired after 40 years of teaching with the last 10 years at Penn State University. Glenn Sartori, Retired Glenn J. Sartori is a freelance writer and co-owner of Text On Demand, a company that specializes in presenting and documenting technical information. Glenn spent 40 years in the aerospace industry at several companies and recently retired from The Boeing Company. Page 15.751.1 Page © American Society for Engineering Education, 2010 Instructional Algorithms Enhance Student Understanding Of PLC Ladder Logic Programming Abstract This paper presents two techniques that helps students transition from relay ladder logic concepts to programmable logic controller (PLC) ladder logic programming. The first technique presents a very structured algorithmic process for the selection of PLC ladder logic input contact configurations for a given control problem. The second technique describes the use of ladder logic building blocks for the commonly used ladder logic instructions in the development of larger PLC ladder logic programs. Students, familiar with relay ladder logic (RLL) control, know that input switch and sensor contacts are wired into the relay logic rung to provide power flow to the output field device, which is wired into the output side of the ladder. If the normally open (NO) contacts of the switch are used in RLL, then the contact symbol for input switch is a NO contact symbol. The field device wired into the output side of the rung is energized or de-energized based on the state of the input contacts. PLC ladder logic inputs are logical representations of memory bits in the input register. Selection of the correct input instruction, examine if closed or examine if open, depends on: the type input field device contact used, NO or NC; the state of the input, active or not active; and the desired state of the output field device, energized or not energized. The selection table process and algorithm presented in the paper leads students to the correct PLC ladder logic input choice. The process also helps them understand the relationship between the physical field device input and output hardware, and the software logic represented in the PLC ladder logic program. As students incorporate PLC ladder logic instructions into a software solution for a sequential control problem, they are faced with PLC instructions that support two or more output options. Timers, for example, have three output options depending on the type of control required by the sequential control problem. The paper describes how instruction options are structured into multi-rung program building blocks with specific control functions. Students are better able to select an instruction, and the instruction output option most appropriate for a control requirement, when they understand the operation of smaller building blocks. Large ladder logic programs are just combinations of these instruction building blocks. Use of both of these student learning techniques in teaching PLC programming has demonstrated their effectiveness in moving students from relay ladder logic to an understanding or PLC ladder logic programming. Page 15.751.2 Page Selecting XIC and XIO Input Instructions Students learning PLCs are often confused when selecting the correct XIC (examine if closed) or XIO (examine if open) input instructions for the PLC ladder rung. An input field device with an NC contact configuration does not necessarily have an examine if open (-| / |-) instruction type representing it in the PLC rung. PLC input rung instructions must be selected using a process that looks at the input field device contact type (NO or NC) and its operational condition (activated or not activated). In addition, the input rung instruction selected, XIC or XIO, is affected by the requirement for the output field device to be either on or off. Therefore, learning a process that will always Figure 1 Process fluid heating tank work is critical. The process system in Figure 1, employed to heat a process liquid, is used here to illustrate the selection of input instructions given a process operational statement. Process input field devices include: • Two float switches – a low level (L1) that is normally open (NO) and a high level (L2) that is normally closed. • A temperature switch – one switch (TS1), which is activated by a temperature over 185° F, that has one normally closed (NC) contact and one NO contact. • Two momentary push button switches – an NC stop (PB1) and an NO start (PB2). The output field devices include: • Two contactors with overloads – one for the heater and one for the pump. • A solenoid operated outlet valve. The process operation for the heater is described as follows: The process liquid is heated when the liquid level is above the low-level float switch, L1, and when the process temperature is below 185° F. The tank is drained when the process temperature is above 185° F, and liquid is above the low-level float switch. The NO contacts of TS1, the temperature switch, are used for control of the drain valve. An energized drain valve opens the drain. The selection table in Figure 2 illustrates a selection strategy for the input PLC temperature switch instruction in the rung that controls the drain valve. When the design is performed, the Page 15.751.3 Page temperature switch requires an instruction on the ladder rung using either an XIC or XIO instruction. The instruction type selected, XIC or XIO, depends on how the control statement Page 15.751.4 represent NO contacts with an activated field device. Note in column 3 that an active NO switch has closed contacts. 3. Determine the desired condition, on or off, for the output field device in column 4. Locate the corresponding row from the two rows selected in step 2. Tank example – Since the outlet valve must be turned on (column 4) or opened, the parameters across row 4 describe the operation of the input and output field devices for this problem. For example, an active NO input field device is turning on the ladder output OTE instruction, so that the output field device is on. Therefore, the closed contacts on the input field device must make the rung true. 4. With the row (4) determined, the type of input instruction required for the problem is listed in column 6. Tank example – The rung input instruction must be examine if closed or XIC (-| |-) for this application. The XIC instruction is true when the NO input field device contact is closed. This true XIC instruction allows power flow to the output coil, which energizes the outlet valve. Figure 3 Ladder logic solution for the process heater tank problem Page 15.751.5 Page The solution for the process tank problem is illustrated in Figure 3. Although the process was demonstrated using input and output field devices, it works equally well with internal input bits and internal output bits. After working through several examples, the process for picking XIC and XIO instructions becomes clear and dependence on the chart in Figure 2 diminishes. The chart illustrates the relationship between inputs, outputs, and input instructions. Standard PLC Ladder Logic Programming Aids Students are often introduced to PLC programming with pedagogy similar to that used for a traditional text based programming languages. Namely, teach the syntax for the instruction when entered into the program code, and describe how values in the affected data registers are modified when the instruction executes. Students are then required to group instructions into code sequences to produce a desired programming result. Following this process for PLC program creation creates Figure 4 PLC TON timer instruction considerable confusion because the structure and syntax of PLC programs, called ladder logic, is entirely different from the syntax of high level languages like C or BASIC. For example, the operation of the PLC on-delay timer instruction (TON) in Figure 4, which starts the timing process when the instruction is active, is introduced by teaching timer parameter selection and action of input and output control bits during a timing cycle. Timer parameters are described as follows: timer name or number (T4:5), time base (0.01 or 1 second per increment), preset (total number of increments), and accumulator value (the number of increments accumulated). At this Figure 5 PLC TON timer in ladder logic point the three timer output bits are introduced: the (EN) enable (a bit to indicate that the timer is active), the (TT) timer timing (a bit to indicate an active timer with an accumulator value less than the preset value), and the (DN) done (a bit to indicate an active timer with an accumulator value equal to or greater than the preset value). Next the instruction is used in a line of code or, in the case of a PLC, the instruction is placed into a ladder logic rung. Figure 5 illustrates the initial programming step for the TON timer Page 15.751.6 Page instructions. The timer instruction is an output; therefore, the instruction is placed on the far right side of the rung. An input instruction is placed on the left side of the rung to establish the input condition necessary to activate the timer. Teaching the TON Instruction Using the traditional programming language instructional approach, lectures on the PLC Figure 6 Two rung timer solution for pump TON timer instruction starts with procedures delay problem for selecting and setting timing parameters, timing diagrams to describe the relationship of timer input and output bits for various true and false conditions, and the addressing syntax for timer data.